Networks of single wall carbon nanotubes (SWCNTs) have shown considerable promise as the active channel material for high performance TFTs fabricated via solution processes. Compared with other competing solution processable semiconducting materials such as conjugated polymers or metal oxides SWCNTs offer significant advantages in terms of higher current densities and mobilities (Cao 2009; Wang 2013; Park 2013). Many SWCNT-based TFTs fabricated to date have utilized thermally grown or vapor deposited inorganic dielectrics such as SiO2, Al2O3 or HfO2 (Snow 2003; Snow 2005; Cao 2008; Okimoto 2010; Takahashi 2011; Sun 2011; Miyata 2011; Wang 2012; Lee 2012). Realization of all-printable devices on plastic substrates requires selection of solution processable gate dielectric materials with good mechanical properties and low curing temperatures. High capacitances (>10 nF/cm2) are required in order to enable low voltage operation. Materials with high dielectric constants (k) are attractive in this regard since target capacitances can be attained with less stringent constraints on film thickness. This is particularly important for all printed devices due to the difficulties of printing thin pinhole free dielectric films. Solid polymer electrolytes (ion gels) offer extremely high capacitances per unit area but slow polarization response will limit their operation at higher frequencies (Ha 2010; Ha 2013). Recently Hersam and co-workers have demonstrated SWCNT TFTs with a high k ZrO2 dielectric deposited by a sol-gel process (Kim 2013), however thermal curing of this material requires 500° C., incompatible with most plastic substrates.
Polymers are attractive candidates for gate dielectric materials in printed devices due to their solution processability, tunable surface chemistry, high degree of bendability, and ability to planarize rough surfaces. Several groups have reported the use of polymer dielectrics to demonstrate low voltage operation of organic field effect transistors (Yoon 2005; Xu 2009; Machado 2012; Xu 2012), but there have been fewer efforts to integrate these materials into SWCNT based TFTs. Bao and co-workers have employed thin (25 nm thick) poly(4-vinylphenol) (PVP) films deposited by spin coating as the gate insulator and demonstrated low voltage operation (<1 V) with mobilities as high as 13.4 cm2/Vs, on/off ratios of 103 and low sub-threshold slopes (130 mV/decade) (Roberts 2009). Polymethyl methacrylate (PMMA) has also been used as gate dielectric for SWCNT TFTs (Tseng 2009; Sun 2013). In order to raise the dielectric constant, BaTiO3 nanoparticles have been incorporated into PMMA and formulated into a commercially available ink suitable for printing. Lau et al. have demonstrated gravure printing of this hybrid ink to fabricate gate dielectric films (k=17) on-top of SWCNT networks to form all-printed top gate TFTs with mobilities of about 4 cm2/Vs (Lau 2013).
Cyanoethylated pullulan is a polymer made by transforming some of the alcohol groups of this polysaccharide into cyanoethyl ether functions. The polar nature of the cyanoethyl groups give the material a high dielectric constant (k˜13), significantly higher than PMMA or PVP. Previously Xu and Rhee have demonstrated its utility as a gate dielectric layer in pentacene TFTs (Xu 2009). Cyanoethylated pullulans, i.e. cyanoethyl dihidroxypropyl pullulan (CEDHPP), have also been disclosed as gate dielectrics in TFTs with organic semiconductors (Moon 2009; Fukui 2007).
Thin film transistors (TFTs) made from solution processes typically have low mobility values and require high voltages for operation. According to the prior art, small molecules, conjugated polymers and SWCNTs may be used to make solution processed TFTs. The use of single-walled carbon nanotubes (SWCNT) as the semiconducting channel material provides improvements, but better dielectric materials are required to lower operating voltages, increase on-state current densities and facilitate enhancement mode operation (ie. transistor off at zero volts). High k dielectrics such as ZrO2 and HfO2 provide improvements over SiO2, but are not readily processable in solution at low temperatures. The use of cyanoethylated pullulans as a gate dielectric in a TFT have been disclosed in patent and publication literature although not demonstrated with SWCNTs as the semiconducting channel. Experiments have shown that applying aqueous dispersions of SWCNT directly on a cyanoethylated pullulan does not work because the SWCNT network does not adhere to the dielectric.
A low temperature solution processable, high quality dielectric capable of adsorbing SWCNT, yielding low voltage operation, high on-state currents and negative threshold voltages (enhancement mode operation) is desired in the art.
There remains a need for solution processable (at temperatures below 200° C.) carbon nanotube-based TFTs having high k gate dielectrics.